Semiconductor light-emitting diodes (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors; for example, binary, ternary, and quaternary alloys of gallium, aluminum, indium, nitrogen, phosphorus, and arsenic. III-V devices emit light across the visible spectrum. GaAs- and GaP-based devices are often used to emit light at longer wavelengths such as yellow through red, while III-nitride devices are often used to emit light at shorter wavelengths such as near-UV through green.
Gallium nitride LEDs typically use a transparent sapphire growth substrate due to the crystal structure of sapphire being similar to the crystal structure of gallium nitride.
Some GaN LEDs are formed as flip chips, with both electrodes on the same surface, where the LED electrodes are bonded to electrodes on a submount without using wire bonds. In such a case, light is transmitted through the transparent sapphire substrate, and the LED layers oppose the submount. A submount provides an interface between the LED and an external power supply. Electrodes on the submount bonded to the LED electrodes may extend beyond the LED or extend to the opposite side of the submount for wire bonding or surface mounting to a circuit board.
FIG. 1 is a simplified cross-section of a GaN LED 10 mounted to a submount 12. The submount may be formed of silicon or may be a ceramic insulator. If the submount is silicon, an oxide layer may insulate the metal pattern on the submount surface from the silicon, or different schemes of ion implantation can be realized for added functionality such as electro-static discharge protection.
Metal pads 14 on the submount are electrically bonded to metal electrodes 16 on the GaN layers 18, where the electrodes 16 are in electrical contact with the n-type and p-type layers of the LED. The bond typically uses gold stud bumps 20. The gold stud bumps 20 are generally spherical gold balls placed at various points between the LED electrodes and the submount metal pads. This is a time-consuming process since the stud bumps must be individually placed. Pressure is applied to the LED structure while an ultrasonic transducer rapidly vibrates the LED structure with respect to the submount to create heat at the interface. This causes the surface of the gold stud bumps to interdiffuse at the atomic level into the LED electrodes and submount electrodes to create a permanent electrical connection.
Other types of bonding methods include soldering, applying a conductive paste, and other means.
Between the LED layers and the submount surface is a large void that is filled with an epoxy to provide mechanical support and to seal the area. The resulting epoxy is referred to as an underfill 22. Underfilling is very time-consuming since each LED must be underfilled separately, and a precise amount of underfill material needs to be injected and prevented from spreading in an uncontrolled fashion onto undesirable surfaces, such as the top of the LED device or pads on the submount where wire bonds must be subsequently applied.
Applicants are developing LED structures with the sapphire substrates removed after the LED structure is bonded to the submount. Since the LED layers are very thin and brittle, the underfill serves the additional purpose to provide the necessary mechanical support to prevent fracturing of the fragile LED layers. The gold stud bumps 20 do not provide sufficient support by themselves to prevent fracturing of the LED layers since, given their limited shape, they are spaced too far apart. The underfill, however, has to flow through a complicated geometry without trapping any bubbles that could result in poorly supported regions. Additionally, underfill materials are typically composed of organic substances and possess very different thermal expansion properties from metal and semiconductor materials. Such spurious expansion behavior is particularly aggravated at high operating temperatures—typical of high power LED applications—where underfill materials approach their glass transition point and begin to behave as glassy substances. The net effect of such mismatch in thermal expansion behavior is to induce stresses on the LED devices that limit or reduce their operability at high power conditions. Lastly, underfill materials have low thermal conductivity properties that result in unnecessarily high temperature operation for the semiconductor devices.
What are needed are techniques for mechanically supporting the thin LED layers during a substrate removal process which, compared to an underfill, (i) provide a lower cost and higher throughput manufacturable solution, (ii) provide more uniform and void free support, (iii) provide a support with more closely matched thermal expansion behavior, (iv) provide a support with high temperature operability, not limited by the glass transition point of organic materials, and (v) provide a support with improved thermal conductivity for superior heat sinking.